Technical Field
This disclosure relates to gas burners in general, and more specifically, to gas burners of multi-burner applications where only one burner contains an igniter and the remaining burners must be lit from the single burner with the igniter using flame carryover. Still more specifically, this disclosure relates to improvements in flame carryover aspects of low NOx burners that reduce the gas used for flame carryover while still providing a robust ignition for all burners.
Description of the Related Art
During the combustion of natural gas, liquefied natural gas on propane, NOx is formed and emitted to the atmosphere with other combustion products. Because these fuels contain little or no fuel-bound nitrogen per se, NOx is largely formed as a consequence of oxygen and nitrogen in the air reacting at the high temperatures resulting from the combustion of the fuel.
Governmental agencies have passed legislation regulating the amount of NOx that may be admitted to the atmosphere by gas furnaces and other devices. For example, in certain areas of the United States, e.g., California, regulations limit the permissible emission of NOx from residential furnaces to less than 40 ng/J (nanograms of NOx per Joule of useful heat generated). Future regulations include plans to restrict NOx emissions from residential furnaces and boilers to less than 15 ng/J.
Gas furnaces often use a particular type of gas burner commonly referred to as an in-shot burner or two-stage burner. Such burners include a burner nozzle having an inlet at one end for receiving separate fuel and primary air streams and an outlet at the other end through which mixed fuel and primary air discharges from the burner nozzle in a generally downstream direction. Fuel gas under pressure passes through a central port disposed at or somewhat upstream of the inlet of the burner nozzle. The diameter of the inlet to the burner nozzle is larger than the diameter of the fuel inlet so as to form an annular area through which atmospheric air (a.k.a. primary air) is drawn into the burner nozzle about the incoming fuel gas.
The primary air mixes with the fuel gas as it passes through the tubular section of the burner nozzle to form a primary air/gas mix. This primary air/gas mix discharges from the burner nozzle and ignites as it exits the nozzle outlet section forming a flame projecting downstream from a flame front located immediately downstream of the burner nozzle outlet and spaced apart from an inlet of the primary heat exchanger. Secondary air flows around the outside of the burner nozzle and is entrained in the burning mixture downstream of the nozzle in order to provide additional air to support combustion as the burning mixture enters the heat exchanger inlet.
In-shot burner designs cannot meet the more stringent NOx emission requirements because of their reliance on secondary air to complete the combustion process. The mixing of air and fuel of such systems produced unacceptably high NOx emissions higher-than the future regulations. In order to comply, the current in-shot burner design is being replaced by burner designs where the air and fuel is fully premixed before combustion, without the use of secondary air. Instead of providing a gap between the burner and heat exchanger which allows for the entrainment of secondary air, the premixed burners are coupled to the heat exchanger inlet. By eliminating the use of secondary air, the premixing of the fuel and air can be controlled and a premixed, lean mixture may be used for combustion which produces less NOx than traditional in-shot burners.
In multi-burner applications such as a typical sectional gas furnaces each heat exchanger is supplied hot combustion products by individual burners. Typically only one burner contains an igniter and therefore, upon ignition, the remaining burners are lit from the single burner with the igniter. Flame carryover is the ability to transfer the flame from one burner to the next. The current industry standard “in-shot” burner uses a small channel between burners where a small flame transfers hot gases to light each successive burner as shown in FIG. 2. This carryover method has proven ineffective when used in combination with premix burners disposed immediately upstream of the heat exchanger.